There are several highly reputable tuning companies and aftermarket upgrade suppliers in our vendors' section - including Mickey's. But diesel tuning isn't a priority for any that I can think of - probably because few D buyers have felt the need to ask for it yet.
My mpg quote is simply the overall consumption read-out off the computer from new and is probably an average of 20 percent in-town, 55 percent of A and B roads and 25 percent of M-ways - all driven briskly as soon as the running-in was done but no so far over the limit as to risk more points!
My Clubman D certainly won't compete with a 320 M-Sport in a traffic-light grand prix - but that's not what Cooper Ds are all about. So try one; you may hate it, but there's no way I'd do a swap for yours!